LED Fabrication via Ion Implant Isolation

ABSTRACT

A semiconductor light emitting diode includes a semiconductor substrate, an epitaxial layer of n-type Group III nitride on the substrate, a p-type epitaxial layer of Group III nitride on the n-type epitaxial layer and forming a p-n junction with the n-type layer, and a resistive gallium nitride region on the n-type epitaxial layer and adjacent the p-type epitaxial layer for electrically isolating portions of the p-n junction. A metal contact layer is formed on the p-type epitaxial layer. In method embodiments disclosed, the resistive gallium nitride border is formed by forming an implant mask on the p-type epitaxial region and implanting ions into portions of the p-type epitaxial region to render portions of the p-type epitaxial region semi-insulating. A photoresist mask or a sufficiently thick metal layer may be used as the implant mask.

RELATED APPLICATIONS

This is a divisional of copending application Ser. No. 11/154,619 filedJun. 16, 2005 and now U.S. Pat. No. ______. U.S. Pat. No. 11/154,619 isa continuation of application Ser. No. 10/840,463, filed May 5, 2004,now U.S. Pat. No. 7,338,822. This application is also related to Ser.No. 10/987,627 filed Nov. 12, 2004 (now abandoned) and to copendingapplication Ser. No. 12/387,882 filed Dec. 4, 2008.

BACKGROUND

The present invention relates to the manufacture and packaging ofsemiconductor light emitting diodes (“LED”). An LED is a semiconductordevice that emits light whenever current passes through it. In itssimplest form, a light emitting diode includes a p-type portion and ann-type portion to define a p-n junction diode. When mounted on a leadframe and encased in an encapsulant (usually a polymer), the overall LEDpackage is also referred to as a “lamp.”

Because of the high reliability, long life and generally low cost ofLEDs, they have gained wide acceptance in a variety of lightingapplications in many fields of application.

LED lamps are extremely tough. They typically do not include glass andavoid filaments entirely. As a result, LED lamps can take abuse farbeyond that of the incandescent lamp and their high reliability cangreatly reduce or eliminate many maintenance factors and costs.

LED lamps can be extremely efficient, e.g., emitting light equal to anincandescent lamp while consuming only 10 percent of the electricity.Many LEDs have life spans of 100,000 hours; i.e. equivalent to over 11years of continuous use. Therefore, from a statistical standpoint, mostLED's will never fail once they are initially tested (typically as partof the production process). LED lamps are excellent for use in unusualor difficult environments such as near explosive gases or liquids.Although individual light choices (solid state versus incandescent orfluorescent) still must be designed and tested for each particular use,as a general rule, LED lights are a safer choice in a wide variety ofapplications.

LED lamps are energy efficient and environmentally friendly. Theyminimize the use of electricity and batteries, and their relatively lowcurrent requirements means they can be solar powered more easily.

The nature, structure and operation of LEDs is generallywell-understood. A conceptual discussion and understanding of the natureand operation of light emitting diodes and the physics and chemistrythat support their operation, can be found for example in textbooks suchas Sze, PHYSICS OF SEMICONDUCTORDEVICES, 2d Ed. (1981) and Sze, MODERNSEMICONDUCTOR DEVICE PHYSICS (1998).

A number of commonly assigned patents and co-pending patent applicationslikewise discuss the theory and nature of light emitting diodes,including but not limited to U.S. Pat. Nos. 6,582,986; 6,459,100;6,373,077; 6,201,262; 6,187,606; 5,912,477; 5,416,342; and 5,838,706;and Published U.S. Applications Nos. 20020093020 and 20020123164. Thecontents of these are incorporated entirely herein by reference.

As all of these sources attest, the color emitted by a light emittingdiode depends upon the nature of the semiconductor material from whichit is formed. As particularly set forth in the commonly assigned patentsand applications, light in the green, blue, violet, and ultravioletportions of the electromagnetic spectrum has higher energy compared withred or yellow light. Such high energy light can typically only begenerated using materials having a wide band gap, that is, a bandgapsufficient to create photons with the required energy. (“Bandgap” is anintrinsic quality of a semiconductor material that determines the energyreleased when a photon is generated in the material.) Silicon carbide,gallium nitride, and other Group III nitrides, as well as certain II-VIcompounds such as ZnSe and ZnS are examples of wide-bandgapsemiconductor materials capable of generating blue, green and/or UVlight. As further set forth in the incorporated references, of thesematerials, gallium nitride and other Group III nitrides have begun toemerge as favorite materials for LED production.

For a number of packaging and use applications, a favored design for alight emitting diode is the “vertical” orientation. The term “vertical”is not used to describe the final position of the overall device, butinstead to describe an orientation within the device in which theelectrical contacts used to direct current through the device and itsp-n junction are positioned on opposite faces (axially) from one anotherin the device. Thus, in its most basic form, a vertical device includesa conductive substrate, a metal contact on one face of the substrate,two or more epitaxial layers on the opposite face of the substrate toform the p-n light-emitting junction, and a top contact on the topepitaxial layer to provide a current path through the layers and theirjunction and the substrate to the substrate contact.

In the latest-generation LEDs produced by the assignee of the presentinvention, e.g., published U.S. Application No. 20020123164, the basicLED structure includes a silicon carbide substrate, an n-type galliumnitride epitaxial layer on the substrate, a p-type gallium nitride layeron the n-type layer, thereby forming a p-n junction and a metal stack onthe p-type layer, which also forms the top contact to the device. It hasbeen found that the emission of light from such devices can be enhancedby carefully selecting the transparency and geometry of the substrate tomaximize the emission of light based upon its expected wavelength andthe index of refraction of the silicon carbide substrate and potentiallythat of the packaging material. Accordingly, in the latest commercialembodiments, the light emitting diode is positioned on a lead frame withthe epitaxial layers of the diode adjacent the lead frame with thesilicon carbide substrate above them. This orientation is sometimesreferred to as “flip chip” or “junction down” and will be discussed inmore detail with respect to the drawings. The leadframe is the metalframe onto which a die is attached and bonded. Parts of the leadframemay become the external connections of the circuit.

Although the “flip chip” design is advantageous, it may result in a verysmall tolerance or space between and among the lead frame, the dieattachment metal, the metal contact layers of the device, and theterminal edges of the epitaxial layers. Because the epitaxial layersinclude and define the p-n junction, the tolerances between the metaland the junction can be as small as 1-5 microns. Accordingly when theLED is mounted in a substrate-up, junction-down orientation on the leadframe, and with a metal (or other functionally conductive material)being used to provide an electrical contact between the lead frame andthe ohmic contact to the p-type portion of the diode, the metal used toattach the LED to the lead frame can inadvertently make contact with then-type layer and form a parasitic (i.e. unwanted) metal-semiconductorconnection known to those skilled in the art as a Schottky contact.

Additionally, the passivation layer (typically silicon nitride) that isoften added to protect the diode can crack following thermal ormechanical stress and thus provide additional possibilities for thedevelopment of undesired contacts to the epitaxial layers of the device.

By way of comparison and explanation, the problem described isessentially non-existent when diodes are positioned on the lead framewith the substrate rather than the epitaxial layers adjacent the leadframe. In such cases, the direct electrical contact between the dieattachment metal and the (typically) n-type silicon carbide substrate isof course desired in order to provide current flow through the substrateand the junction.

In a typical LED manufacturing process, epitaxial layers of one or moresemiconducting materials are grown on a semiconductor substrate wafer.Such wafers are typically between 2 inches and 4 inches in diameter,depending upon the semiconductor materials being used. Becauseindividual LED die are typically quite small (e.g., 300×300 microns), alarge quantity of LED die may be formed on a substrate wafer and itsepitaxial layers in a geometric grid pattern. In order to successfullyproduce individual devices, the LED die in the grid must be separatedfrom one another, both physically and electrically. Once the LEDs havebeen formed on a wafer, they are then separated into individual die, orgroups of die, using well understood separation techniques such assawing, scribe-and-break or the like.

The process of die separation may be harmful to exposed p-n junctionregions. Therefore, prior to separation, it is known to isolateindividual die while they remain on the wafer. The most typical methodof isolation, which also serves to clearly define the devices and thelocation for their ohmic contacts, is to carry out one or morephotolithography steps and etching the epitaxial layers to define ajunction-containing mesa for each device or device precursor.

Although photolithography is a useful technique in semiconductor designand manufacturing, it requires specific equipment and materials and addsprocess steps. For example, a typical photolithography process caninclude the steps of adding a layer of photoresist (typically a polymerresin sensitive to light) to a semiconductor structure, positioning amask over the photoresist, exposing the photoresist to a frequency oflight to which it responds (by undergoing a chemical change; usually itssolubility in a particular solvent), etching the photoresist to removethe exposed or unexposed pattern (depending upon the resist selected),and then carrying out the next desired step on the remaining pattern. Inparticular, when the purpose of the patterning step is to define an etchpattern in a GaN-based layer, GaN's chemical, physical, and thermalstability (which are favorable characteristics in finished devices) cancause additional difficulties if the etchant removes the resist beforefully removing the desired pattern of material.

Accordingly, forming mesa-type LEDs that include a top contact metallayer will typically require at least two full sets of these steps; oneset for patterning and etching the mesa and another set for patterningand depositing the metal contact layer.

Therefore, improvements in isolating devices from one another canprovide corresponding improvements in the structure and performance ofLEDs and LED layers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional diagram of an LED according to the prior artmounted on a lead frame in a flip-chip orientation.

FIG. 2 is a cross-sectional diagram of the basic elements of an LEDbefore and after a mesa is formed and a metal contact is added.

FIGS. 3 and 3A are respective a cross-sectional and top plan diagrams ofan LED and schematically illustrating implantation according toembodiments of the present invention.

FIG. 4 is a cross-sectional diagram of an LED according to the inventionmounted on a lead frame in a flip-chip orientation.

FIGS. 5(A) through 5(E) are progressive schematic cross-sectional viewsof the method steps of embodiments of the invention.

FIGS. 6(A) through 6(D) are progressive schematic cross-sectional viewsof the method steps of further embodiments of the invention.

FIG. 7 is a cross-sectional illustration of embodiments of the inventionpackaged in lamp form.

DETAILED DESCRIPTION

The present invention now will be described more fully hereinafter withreference to the accompanying drawings, in which preferred embodimentsof the invention are shown. This invention may, however, be embodied inmany different forms and should not be construed as limited to theembodiments set forth herein; rather, these embodiments are provided sothat this disclosure will be thorough and complete, and will fullyconvey the scope of the invention to those skilled in the art. In thedrawings, the thickness of layers and regions are exaggerated forclarity. It will be understood that when an element such as a layer,region or substrate is referred to as being “on” another element, it canbe directly on the other element or intervening elements may also bepresent. In contrast, when an element is referred to as being “directlyon” another element, there are no intervening elements present.Moreover, it will be understood that when a first element or layer isdescribed as “in electrical contact” with a second element or layer, thefirst and second elements or layers need not be in direct physicalcontact with one another, but may be connected by intervening conductiveelements or layers which permit current flow between the first andsecond elements or layers.

FIG. 1 is a cross-sectional schematic view of a light emitting diodebroadly designated at 20 and illustrated in a manner that illustratesthe potential problems than can arise when light emitting diodes mountedin certain orientations. Although the light emitting diode 20 is shownin fairly simplified fashion, it will be understood by those of ordinaryskill in this art that the device 20 can be more sophisticated (i.e.include more elements) than illustrated herein. In the presentcircumstances, however, the invention can be clearly understood usingbasic illustrations. An exemplary diode of the type illustrated at 20and some of its variations are also described in commonly assigned andco-pending application Publication No. US20020123164, the contents ofwhich are incorporated entirely herein by reference.

The diode 20 includes a substrate 21 which may be n-type silicon carbidehaving a polytype selected from the group consisting of the 2H, 4H, 6H,8H, 15R, and 3C polytypes of silicon carbide. The diode portion of thedevice 20 is illustrated by the respective n-type gallium nitride region22 and p-type gallium nitride region 23, which together define a p-njunction 24. Region 22 and region 23 may each comprise a single layer ora group of related layers having different compositions, thicknesses,dopant concentrations or other qualities. The diode 20 also includes anohmic contact 25 to the p-type gallium nitride epitaxial region 23 and adie attach metal 26 in electrical contact with the ohmic contact. Asdescribed in U.S. patent application Ser. No. 10/200,244 which isincorporated herein by reference as if fully set forth herein the dieattach metal 26 may physically contact a bond pad (not shown) such as agold or silver layer. Moreover, reflector, barrier, and other metallayers (not shown) may be formed between die attach metal 26 and theohmic contact 25. Ohmic contact 15 may be formed on substrate 21 to forma vertical device as described above, and a wire lead 29 may beconnected to contact 15 for connecting the device to an externalcircuit. Moreover, in the device illustrated in FIG. 1, a passivationlayer 5 comprising an insulating material such as silicon nitride orsilicon dioxide covers or an insulating polymer such as polyamide andprotects the exposed surfaces of the epitaxial regions 22, 23.

The diode 20 is typically mounted on a metal or metallized lead frame27, which provides an electrical contact between the diode 20 and anexternal circuit. As set forth in the background, in typicalenvironments, the die attach metal 26 is formed of a metal that melts ata relatively low temperature, e.g., lower than the ohmic contact 25 andlow enough such that other package components are not damaged during thedie attach process. Accordingly, die attach metal 26 may comprise asolder such as tin or an alloy such as gold/tin. The die attach metalpermits the diode 20 to be mounted on the lead frame quickly and easilyvia soldering or thermosonic bonding. In this regard, FIG. 1 alsoillustrates that if the die attach metal is formed imprecisely (which isalways a possibility given the very small scale and size of mostdevices) a portion 30 of the die attach metal 26 can extend beyond thedesired contact with the ohmic contact 25 and can contact the p-typegallium nitride region 23 or the n-type gallium nitride region 22. Inthese circumstances, the imprecise or unwanted portion 30 of the dieattach metal may contact n-type gallium nitride layer 22 and can form anunwanted and parasitic Schottky diode with the n-type layer 22, or if itextends far enough, with the n-type silicon carbide substrate 21.

FIG. 2 illustrates a conventional manner of defining or isolating ajunction in a light-emitting diode. As in the case of FIG. 1, the diode(broadly designated at 32) has been illustrated in its most fundamentalaspects and can include additional elements that for the sake of clarityare not illustrated in FIG. 2. As in the case of the diode 20 in FIG. 1,the diode 32 includes an n-type silicon carbide substrate 33, an n-typeepitaxial layer of gallium nitride 34 on the substrate 33, and a p-typelayer of gallium nitride 35 on the n-type layer, and defining the p-njunction 36. In order to define and isolate the junction, the epitaxiallayers 34 and 35 are typically etched to form the mesa structureillustrated in the right-hand portion of FIG. 2, which also shows anohmic contact 37 to the p-type epitaxial layer 35. In manycircumstances, the junction 36 within the mesa formed by etching thelayers 34 and 35 is covered with silicon dioxide or some otherappropriate insulating or dielectric material to help protect thejunction 36 from external contamination and/or damage during dieseparation, packaging, or other processing steps, or during operation.

As recognized by those of skill in this art, in order to form the layers35 and 36 into a mesa, a number of additional steps must be carried out.These typically include a masking step to define the pattern for theetch, which in turn includes the steps of laying down the mask(typically a photoresist), placing the optical pattern over thephotoresist, exposing, and developing the resist, removing the developed(or as the case may be, undeveloped) portions of the mask and etchingthe underlying epitaxial layers, then removing the photoresist. Althoughthese steps are now common in the semiconductor industry and can becarried out with a high degree of precision, they add additionalengineering and cost factors to the overall manufacturing process, andfor well-understood reasons, each additional manufacturing step cancreate a small lack of tolerance or precision in the end device.

FIG. 3 is a broad schematic illustration of a junction-defining methodof embodiments of the present invention. The method comprises implantingions represented schematically by the arrows 40 into an epitaxial layer41 in the diode broadly designated at 42 adjacent the p-n junction 43.Epitaxial layer 41 has a first conductivity type (illustrated in FIG. 3as p-type). The type and amount of the implanted ions increase theresistivity of implanted region 44 and may render the implanted region44 highly resistive and/or semi-insulating. In this context, “highlyresistive” means that the material is sufficiently resistive such thatnegligible current flow occurs when a voltage bias is applied to theanodes of adjacent die on a wafer. That is, the material in implantedregions 44 is considered highly resistive for purposes of this inventionif it substantially electrically isolates adjacent die. In someembodiments, the resistivity of the implanted regions 44 is at least2000 Ω-cm. If the resistivity of the material is sufficiently high, thematerial may be considered semi-insulating (or “i-type”) material ratherthan p-type or n-type semiconductor material. In general, materialhaving a resistivity in excess of 1×10⁵ Ω-cm at room temperature may beconsidered semi-insulating for purposes of this invention.

The ions 40 may be implanted in a pattern that defines an implantedperimeter 44 as more clearly illustrated in FIG. 3A. The perimeter 44may be sufficiently doped with the implanted ions 40 to be highlyresistive to thereby isolate or define the junction 43 with theimplanted perimeter 44. FIG. 3 also illustrates the n-type layer 45, thesubstrate 46, and the metal contact 47. The dashed lines in theepitaxial layer 41 also help illustrate the location of the perimeter inthe view of FIG. 3.

In some embodiments, substrate 46 may comprise n-type silicon carbidehaving a polytype selected from the group consisting of the 2H, 4H, 6H,8H, 15R, and 3C polytypes of silicon carbide. It will be appreciated bythose skilled in the art, however, that substrate 46 could compriseanother material such as sapphire, gallium nitride, aluminum nitride oranother suitable material such as MgO, spinel, silicon or ZnO. Moreover,the substrate 46 may be conductive to permit the formation of a verticaldevice, or the substrate 46 could be insulating or semi-insulating.

In some embodiments of the invention, metal contact 47 may serve as theimplant mask for implanting ions 40 into the device. For example, ifmetal contact 47 comprises a metal stack with sufficient thickness toprevent ions 40 from reaching layer 41, then it may be possible to avoiddepositing a separate implant mask.

As FIG. 3 schematically illustrates, in one embodiment, the methodcomprises implanting ions such as nitrogen or phosphorus in the p-typelayer of gallium nitride. Other ions can be used to increase theresistivity of the region via implantation, including hydrogen, helium,aluminum and N₂. Other aspects of the method will be illustrated withrespect to FIG. 5.

The implantation can be carried out in conventional fashion and at roomtemperature. As presently best understood (and without being limited bya particular theory), the implanting ions create damage within the GaNto produce deep levels within the bandgap. These in turn trap freecarriers in the GaN thus rendering the material highly resistive.

Light-emitting diodes in Group III nitrides such as GaN are not limitedto using n-type substrates and p-type top layers. There are, however, anumber of reasons, well understood in this art, as to why n-type SiCsubstrates are more commonly used. Thus, the invention could alsoinclude implanting an n-type layer to increase its resistivity. Becausethe use of n-type substrates is more common, however, most of thedescription herein will refer to such structures.

Although light emitting diodes that incorporate two layers (n andp-type) of gallium nitride are illustrated, those familiar with and ofordinary skill in this art will recognize that the diode 42 can includeone or more quantum wells, or superlattice structures or both and thatthe active layer or layers can include a greater range of the Group IIInitride compounds than gallium nitride standing alone. These variations,however, need not be elaborated in detail in order to clearly understandthe invention, and thus, they are not discussed in detail herein. Thus,the relevant portions of more elaborate devices may also be referred toas, “active layers,” “diode portions,” “diode regions,” or “diodestructures,” without departing from the scope of the present invention.

FIG. 4 is a schematic cross sectional diagram similar to FIG. 1, butillustrating a diode broadly designated at 50 that incorporates theimplanted perimeter region 54 of the present invention. The diode 50includes a substrate, 51 which in some embodiments is n-type siliconcarbide.

An n-type gallium nitrite epitaxial layer is on the substrate 51, andappears underneath the substrate 51 in the “flip-chip” orientationillustrated in FIG. 4. The p-type layer 53 is adjacent the n-type layer52 and the two layers define a p-n junction 58 between them. The diodealso includes the ohmic contact 55, and the die attach metal portion 56.

Because the diode has been implanted in the manner just described withrespect to FIG. 3, it includes the highly resistive perimeter portions54 shown adjacent the dotted lines in FIG. 4. A portion 60 of less thanideally placed die attachment metal is also illustrated 54 on the leadframe 57. As thus illustrated, the invention provides severaladvancements. First, the geometric spatial area available for the metalportion 60 is now more limited because the implanted regions 54 of thediode 50 do not require the multiple steps necessary to form a mesa.Thus, the p-type layer and its insulated portions 54 provide anadditional geometric, spatial blocking of the excess metal 60.Additionally, because the implanted portions 54 are highly resistive,there is little or no electrical interaction between the excess metal 60and the implanted perimeter portions 54. Finally, the amount of excessundesired metal 60 that would be required to reach the n-type epitaxiallayer 52 is much greater. Stated differently, the invention provides agreater margin for error when using a standard or defined amount of dieattachment metal 56, 60. As noted earlier with respect to FIG. 1, inoperation and practice, the diode 50 would have both anode and cathodeconnections to the lead frame but the details of these connections havebeen eliminated to clarify the illustration of the invention.

Further to some additional details of the invention, the substrate 51 isillustrated in FIG. 4 as being formed of conductive silicon carbide, butcan also comprise semi-insulating silicon carbide or sapphire (which isinsulating), because the advantages of the invention are based uponadvantages provided in the epitaxial layers. Thus, although aninsulating or semi-insulating substrate requires a slightly differentgeometry than a conductive SiC substrate for packaging purposes, theprinciples with respect to the invention and the epitaxial layers remainthe same. Furthermore, for a light emitting diode, the substrate 51 ispreferably substantially transparent to the light emitted by thejunction 58 when a potential difference is applied to the device.Because silicon carbide emits in the higher energy portions of thevisible spectrum, the substrate is preferably substantially transparentto light having wavelengths of between about 390 and 550 nanometers, andmore preferably between about 485 and 550 nanometers. U.S. Pat. No.5,718,760 and its siblings Nos. 6,025,289 and 6,200,917, describetechniques for producing colorless SiC. These patents are commonlyassigned with the present invention, and are incorporated entirelyherein by reference.

As stated above, the semiinsulating border portions 54 have sufficientresistivity to preclude Schottky behavior when the border 54 is incontact with the metal 56, 60 that would otherwise produce Schottkybehavior if the border 54 were n-type.

As illustrated in FIG. 7, a light emitting diode 78 of the invention andof the type illustrated in FIG. 3 or FIG. 4 can be mounted in a packagecomprising a header 73, electrical leads 72, and an encapsulant 74 (suchas epoxy) which may be molded in the shape of a lens 75. LED 78 may bemounted in a conventional substrate-down orientation or in a “flip-chip”orientation with the epitaxial layers adjacent the header 73. Theresulting LED lamp 79 can be incorporated as part of a display or can beused as an indicator light, backlight or other application.

The ohmic contacts to the p-type layer are typically selected from thegroup consisting of platinum, nickel, gold, titanium, aluminum, silver,and combinations of these, and when an ohmic contact is made to thesilicon carbide substrate, it is usually selected from the groupconsisting of nickel, platinum, palladium, aluminum, titanium, andcombinations thereof. Because the ohmic contacts to the substrate (notillustrated in FIG. 4) are visibly located in the direction in whichlight is desirably emitted, and because larger contacts havecurrent-spreading advantages, the ohmic contact is preferably selectedand formed to be as close to transparent as possible, typically with anadditional smaller bonding pad being added to enhance the flow ofcurrent to the appropriate circuit.

In a similar manner, the ohmic contacts 55 and the die attach metal 56can be selected to act as a mirror or reflector to enhance the eventualoutput or the diode 50 when packaged and in use. Alternatively, andadditional metal layer (or layers) can be included for this purpose.Moreover, other metal layers may be included in the metal stack forother purposes, such as barrier layers for preventing diffusion ofcontaminants and bonding layers for bonding external contacts to thedevice.

As noted above with respect to the basic illustration of the device andwith respect to the method, the structure of the invention is notlimited to the schematic illustration of FIG. 4. For numerous reasons, abuffer layer is often included as part of the structure between thesilicon carbide substrate and the first gallium nitride (or other GroupIII nitride) layer. In many cases, the buffer layer can comprisealuminum nitride, or a graded layer of aluminum gallium nitride (AlGaN)that progresses from a higher aluminum concentration near the siliconcarbide substrate to a higher gallium nitride concentration at itsinterface with the gallium nitride epitaxial layer. Other structuralportions that can be incorporated into diodes of this type and withwhich the invention is particularly suitable include superlatticestructures for enhancing the overall crystal stability of the device,quantum wells for enhancing the output of light or tuning it to aparticular frequency, or multiple quantum wells for enhancing thebrightness of the device by providing the additional number of activelayers and the relationships between them. In addition, it may bedesirable to passivate the exposed surfaces of the epitaxial layers 52,53 of the device 50 for environmental protection. As described above,such passivation may comprise silicon dioxide or silicon nitridedeposited via PECVD, sputtering, or other suitable passivationtechnique.

FIG. 5 illustrates some of the method aspects of the invention. In abroad sense, the method electrically defines or isolates a p-n junctionin a diode, most preferably in a Group III nitride or galliumnitride-based diode, to minimize or avoid undesired electrical contactsand pathways when the diode is mounted for use and/or to prevent damageto the p-n junction during die separation. In this aspect, the methodcomprises depositing an ohmic metal contact layer on a central portionof a p-type epitaxial layer of gallium nitride that is part of a p-njunction, patterning the metal layer by applying an etch mask on themetal layer and removing a portion of the etch mask and the metal layer,and then implanting ions into the perimeter portions of the p-typeepitaxial layer that are not covered by the etch mask.

In a slightly more detailed aspect, the method can comprise masking aportion of the ohmic metal layer (and potentially a bond pad on theohmic contact), removing the remaining exposed ohmic metal contact layerfrom the epitaxial layer, implanting the exposed portions of epitaxiallayer with atoms sufficient to increase the resistivity of the exposedportions (and potentially render the exposed portions semi-insulating),and removing the mask from the ohmic contact (and the bond pad) tothereby produce high-resistivity portions of the p-type layer. In thisaspect, the method can comprise depositing the ohmic contact and thebond pad prior to the masking step, and masking the ohmic contact metalwith the photoresist.

Turning to FIG. 5 in more detail, it illustrates a device precursorbroadly designated at 60. The diode precursor 60 includes a substrate 61(which in certain embodiments comprises n-type SiC), an n-type galliumnitride epitaxial layer 62 on the substrate 61, a p-type gallium nitrideepitaxial layer 63 on the n-type layer 62, an ohmic contact layer 65 onthe p-type layer 63, and a metal bond pad 66 on the ohmic contact layer65. The various steps of forming the substrate and epitaxial layers anddepositing the ohmic contact and bond pad are generally well understoodin the art and will not be described in detail herein. Representativedescriptions are included in issued patents including but not limited tocommonly assigned U.S. Pat. Nos. 6,297,522; 6,217,662; 6,063,186;5,679,153; 5,393,993; and 5,119,540.

FIG. 5(B) illustrates the diode precursor 60 after a photoresist hasbeen deposited, masked, and patterned to form the photoresist portion 67on all of the bond pad as illustrated in FIG. 5(B) and some, but notall, areas of the ohmic contact 65. The precursor in FIG. 5(B) is thenetched to remove the portions of the ohmic contact layer 65 that are notcovered by the photoresist 67. The etching can be carried out in anyappropriate manner, with reactive ion etching (RIE) using achlorine-based plasma being a possible method. The etching results inthe structure shown in FIG. 5(C) in which the size of the ohmic contacthas been reduced to an area represented as 65(a). FIG. 5(C) alsoillustrates that with a portion of the ohmic contact layer 65 removed,portions of the p-type epitaxial gallium nitride layer are uncovered.

FIG. 5(D) thus shows the implantation (schematically) of ions 70 to formthe highly resistive regions 71 in the p-type gallium nitride layer 63that define and isolate the junction 64. In some embodiments, the highlyresistive implanted regions 71 are semi-insulating.

Alternatively, the implant step illustrated in FIG. 5(D) may beperformed prior to the step of etching the ohmic contact layer 65 if theohmic contact layer 65 is sufficiently thin so as not to substantiallyinterfere with the implantation step.

When the photoresist 67 is removed, the precursor structure illustratedin FIG. 5(E) results. The precursor structures may then be separatedinto individual die.

Because the photoresist 67 serves as a mask for two steps (etching theohmic contact layer 65 and implanting the ions 70), the method of theinvention reduces fabrication cycle time, wafer handling and chemicalconsumption, and likewise reduces the yield loss otherwise associatedwith the masking and etching procedures.

Another embodiment of the invention is illustrated in FIG. 6(A)-(D). Inthis embodiment, a wafer 80 comprising a substrate 81 and epitaxialregions 82 and 83 is provided. As illustrated in FIG. 6(A), a metalstack 86 is formed on epitaxial region 83. As discussed above, epitaxialregion 83 has a first conductivity type and epitaxial region 82 has asecond conductivity type opposite the first conductivity type. Eachepitaxial region 82 and 83 may comprise one or more layers having thesame conductivity type. Metal stack 86 may comprise a number of metallayers, each of which has a particular function. For example, metalstack 86 may comprise a layer of metal for forming an ohmic contact withepitaxial region 83. Metal stack 86 may also comprise reflector,barrier, adhesion, bonding, and/or other layers.

As illustrated in FIG. 6(B), etch mask 85 is deposited on metal stack 86and patterned via photolithography to form openings that selectivelyreveal surface portions 86A of metal stack 86. Alternately, metal stack86 could be formed by photolithography, deposition and liftofftechniques which are well known in the art. That is, metal stack 86could be formed by applying a blanket photoresist to the surface of theepitaxial region 83, patterning the photoresist by exposing anddeveloping it, depositing the metal as a blanket layer and lifting offthe unwanted metal.

Turning now to FIG. 6(C), metal stack 86 is selectively etched to revealsurface portions 83A of epitaxial region 83. Etch mask 85 is thenremoved by conventional methods. Ions 87 are then implanted into theexposed portions of epitaxial region 83 to render the implanted regions84 highly resistive in the manner described above and to define p-njunction regions 88 within the structure. In this embodiment, thepatterned metal layer 86 serves as the implant mask.

Finally, as illustrated in FIG. 6(D), the etch/implant mask 85 isremoved and individual die 89 are separated using conventionaltechniques such that defined p-n junction regions 88 are spaced apartfrom the sidewalls 90 of the die, and are thereby physically andelectrically isolated.

EXPERIMENTAL

The following implantation procedures were carried out in evaluating thepresent invention:

In each of the following evaluations, an LED precursor comprising ann-type silicon carbide substrate, an n-type epitaxial region and ap-type epitaxial region was provided. The p-type epitaxial regioncomprised GaN/AlGaN layers doped with Mg at a carrier concentration ofabout 1 to 5×1017 cm-3 and had a total thickness of about 210 nm. In afirst evaluation, successive doses of 20 keV of monovalent nitrogen(N+1) at a dosage of 1013 per square centimeter (cm-2), 125 keV of N+1at a dosage of 1.4×1013 cm-2, and 125 keV of divalent nitrogen (N⁺²) ata dosage of 2×10¹³ cm⁻² were carried out.

In a second evaluation, N⁺¹ nitrogen was implanted at 20 keV at a dosageof 1013 cm⁻² followed by a dosage at 125 keV of monovalent nitrogen at1.4×1013 cm⁻².

In a third evaluation, the first dosage was carried out at 20 keV usingN+¹ at a dosage of 1013 cm⁻², followed by 125 keV of at a dosage of1.4×1013 cm⁻² followed by 190 keV of N+1 at 1.7×10¹³ cm⁻².

Of the three conditions, all produced junction isolation and definition.Junction isolation was verified by probing adjacent metal stacks andperforming a continuity measurement. No measurable current was observedprior to breakdown of the p-n junction. Implanted helium and hydrogenhave also created resistivity conditions leading to the potentialconclusion that implanting almost any ion into p-type gallium nitride atroom temperature will cause the material to become dramatically moreresistant.

The inventors, however, do not wish to be bound by any particulartheory. Accordingly, the potential conclusions discussed herein areoffered for the sake of illustration rather than limitation.

These implantation steps rendered the edges of the junction insulatingand inert (as desired). The implantation step is also favorable forp-type gallium nitride because of the low hole concentrations (about1×10¹⁷ cm⁻³) that p-type gallium nitride demonstrates. The inertcharacteristic produced by the implant of the invention appears to bestable to temperatures approaching 900° C. Thus, junction isolation bythis technique appears to lend itself to any nitride device thatincorporates a p-type layer. For this reason, nitride based lightemitting diodes on sapphire as well as on silicon carbide and othersubstrates appear to benefit equally from the use of implantation forjunction isolation.

The following evaluations were carried out on commercial dies from Cree,Inc.

The standard implant condition was (1) below. Other conditions (2)-(5).were also examined. The (2) condition was intended to evaluate thenumber of carriers that are eliminated per implanted ion. Conditions (3)and (5) were intended to evaluate the effect of implanting on the p-sideof the device only. Condition (4) evaluated the efficiency of trappingcarriers similar to the (1) and (2) comparison. In carrying out theimplantations, it was determined that higher energy implants were neededfor Cree's green LEDs as compared to Cree's blue LEDs.

Implant Conditions Explored On Blue (indicating successive doses):

1e13@20 keV, 1.4e13@125 keV, 1.7e13@ 190 keV (standard triple dosecondition)

1e12@20 keV, 1.4e12@125 keV, 1.7e12@190 keV (standard condition@ 1/10dose)

1.4 e13@125 keV (single dose from standard condition of #1)

1.4 e12@125 keV ( 1/10 dose of #4)

1e13@30 keV, 1.4e13 @100 keV

For Cree's blue light emitting diodes and based on TRIM simulations the190 keV implant of (1) and (2) goes far deeper than the p-n junctionplacing most of the 190 keV implanted nitrogen on the n-side. The singledose at 125 keV of (3) and (4) and the double dose of (5) wereconsidered. The 100 and 125 keV energies place the peak of the nitrogenconcentration at or about the p-n junction which is 1500-1800 A belowthe surface. The simulated peak for the 100 and 125 keV implants isabout 1600 and 2000 A, respectively. The GaN material is converted fromlow resistivity to high resistivity.

P-type GaN is about 1200-1500 A thick with a free hole concentration ofabout 2-5 E17/cm³

P-type AlGaN is about 300 A thick with a hole concentration of about5-20 E16/cm³

Condition (1) always worked for the blue LEDs (i.e., desired isolationand electrostatic discharge (ESD) yield).

Condition (2) successfully isolates devices, but the ESD yield is poor.

Condition (3) isolates the devices with good ESD yield.

Condition (4) does not quite isolate the devices and does not offer goodESD yield.

Condition (5) isolates the devices with good ESD yield.

For Cree's Green light emitting diodes, each of conditions (1)-(5)created high resistivity p-type material and isolated adjacent devices,but none produced good ESD yield. The addition of a further 230 keVnitrogen implant with a dose of 2 E 13/cm², however, worked well forisolation and ESD yield.

Production Recovery Process: Other devices were passivated postfabrication in which cases the device had a silicon nitride coveringabout 1600 A thick. The best conditions for blue remained condition (1)above, but with an extra 30 keV of energy to penetrate the passivationlayer. Similar results were obtained for the green LEDs by adding theextra 30 keV to the added 230 keV implant at 2 E13/cm². The doses wereunchanged.

Estimation of resistivity: The resistivity was estimated to be greaterthan 2×10³ ohm-cm. This was done by evaluating the current flowingbetween two adjacent pads with an applied voltage of 2 Volts. Theresulting current was unmeasurable (<50 nA). This corresponds to aresistance greater than 40×10⁶ ohms. The distance between the pads is 70microns and the width is 230 microns. The number of squares is thenabout 0.3 between them. The effects of fringing were ignored to give theestimate a worst case lower limit on resistivity. This gives a lowerlimit on the sheet resistance of 133×10⁶ ohms per square. Taking thethickness to be 0.15×10⁻⁴ cm thick yields a resistivity of greater than2000 ohm-cm.

Embodiments of the invention have been set forth in the drawings andspecification, and although specific terms have been employed, they areused in a generic and descriptive sense only and not for purposes oflimitation, the scope of the invention being defined in the claims.

1. A semiconductor light emitting diode comprising: a semiconductoractive structure for providing an optical emission when current isinjected therethrough; a p-type epitaxial layer of a Group III nitrideon said active structure; a highly resistive gallium nitride border onsaid active structure and surrounding said p-type layer for electricallyisolating portions of said active structure; and respective ohmiccontacts to said diode for injecting current through said diode and saidactive structure and generating a desired emission therefrom.
 2. A lightemitting diode according to claim 1 wherein said active structurecomprises one or more Group III nitride compositions.
 3. A lightemitting diode according to claim 1 wherein said highly resistive borderhas sufficient resistivity to preclude Schottky behavior when saidborder is in contact with a metal.
 4. A light emitting diode accordingto claim 1 and further comprising at least one metal contact layer onsaid p-type epitaxial layer.
 5. A light emitting diode according toclaim 1 wherein said highly resistive border has sufficient resistivityto preclude Schottky behavior when said border is in electrical contactwith said metal contact layer.
 6. A light emitting diode according toclaim 1 mounted on a lead frame with said ohmic contacts in respectiveelectrical contact with said lead frame.
 7. A light emitting diodeaccording to claim 1 wherein said ohmic contact to said p-type layer isselected from the group consisting of platinum, nickel, gold, titanium,aluminum, silver, and combinations thereof.
 8. A light emitting diodeaccording to claim 1 further comprising: a semiconductor substrate; anda buffer layer on said substrate; and with said semiconductor activestructure on said buffer layer.